1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine, which is for controlling the valve timing of the internal combustion engine.
2. Description of the Related Art
FIG. 12 is a conceptual illustration of a configuration of an internal combustion engine including a valve timing mechanism, disclosed in Japanese Patent Application Laid-open No. 6-299876.
As FIG. 12 shows, an internal combustion engine 1, including a conventional valve timing control system, is equipped with an air cleaner 2, an air flow sensor 3, a throttle valve 4, a throttle sensor 5, an intake pipe 6, an injector 7, an ignition plug 8, an exhaust pipe 9, an O.sub.2 sensor 10, a catalytic converter 11, a sensor plate 12, a crank angle sensor 13, a sensor plate 14, a cam angle sensor 15, an oil control valve 16, an ECU 17 and an ignition coil 18.
Moreover, as shown in FIG. 13, a housing 21, a rotor 22, a retarding chamber 23, and an advancing chamber 24 constitute a VVT (Variable Valve Timing) actuator 20.
The air cleaner 2 is installed at an opening of the intake pipe 6 to clean the air the internal combustion engine 1 intakes. The air flow sensor 3 is installed on the downstream side of the air cleaner 2 to sense the air intake amount into the internal combustion engine 1. The throttle valve 4 is opened and closed in connection with the accelerator pedal to adjust the air intake amount. The throttle sensor 5 detects the opening degree of the throttle valve 4.
In such an internal combustion engine, when the driver steps on the accelerator pedal, the throttle valve 4 opens/closes, so that air is mixed with a fuel injected from the injector 7 and this air-fuel mixture is introduced into cylinders. The air-fuel mixture is then ignited by the ignition plug 8 to push the pistons downward by the combustion thereof, thereby causing the crank shaft to rotate. The rotation of the crank shaft is derived as the output of the internal combustion engine.
With this operation of the internal combustion engine, the O.sub.2 sensor 10 detects the residual oxygen amount in the exhaust gas. The catalytic converter 11 simultaneously removes THC, CO and NOx which are harmful gases.
FIGS. 14 and 13 are enlarged illustrations of an essential portion of the VVT actuator.
In FIG. 13, the VVT (Variable Valve Timing) actuator 20 is situated on the intake side, and is composed of the housing 21, the rotor 22, the retarding chamber 23 and the advancing chamber 24.
The rotor 22 is fixedly fitted on a cam shaft (not shown) to keep a constant positional relationship (for example, the positional relationship shown in FIG. 13) with respect to the housing 21.
A timing belt, timing chain or the like (not shown) is set on the housing 21. This timing belt or the like is also placed on a crank shaft (not shown).
With this structure, the rotation of the crank shaft causes the rotation of the cam shaft through the timing belt or the like.
Furthermore, the oil control valve 16 controls the oil pressure to be applied to the VVT actuator 20 fitted to the cam shaft.
Thus, in order to vary the valve timing in the internal combustion engine, the ECU 17 controls the VVT actuator 20 through the oil control valve 16 to adjust the amount of the lubricating oil to be supplied to the retarding chamber 23 and the advancing chamber 24.
The ECU 17 shifts the relative position of the rotor 22 with respect to the housing 21, for example, from the position shown in FIG. 13 to the position shown in FIG. 14, thereby changing the valve timing.
FIG. 15 is a graph showing the characteristics of the relationship between valve timing and valve overlap. In this case, the term "valve overlap" signifies the overlap between the time period during which the intake valve is in the open condition and the time period during which the exhaust valve is in the open condition.
For instance, in order to retard the valve timing of the intake valve, the oil control valve 16 supplies the oil to the retarding chamber 23. At this time, the rotor 22 is rotated counterclockwise with respect to the housing 21, and the valve timing of the intake valve is retarded (in the direction indicated by arrow A in FIG. 15), so that the valve overlap decreases.
On the other hand, if the valve timing of the intake valve is advanced (in the direction indicated by the arrow B in FIG. 15), the valve overlap increases.
Furthermore, in the case of retarding the valve timing of the intake valve to a maximum, the housing 21 is brought into contact with the rotor 22 and is fixed at the position (see FIG. 14) where it stops mechanically, this being the position where the valve overlap assumes the minimum value.
In the following description, the advance amount in the case where the valve timing of the intake valve assumes this position will be referred to as a maximum retardation value, and in this case the valve timing of the intake valve is expressed as being at the maximum retardation position.
In the valve timing control for the internal combustion engine, the substantial advance amount (which will be referred to hereinafter as a VVT control variable) by the VVT mechanism is determined with the aforesaid maximum retardation value being employed as a reference. Moreover, this valve timing control is implemented by the ECU 17. The optimal valve timing required for the internal combustion engine varies according to the operating conditions. Therefore, the ECU 17 always controls the valve timing according to the operational conditions.
For instance, a ROM of the ECU 17 retains a two-dimensional map for determining a required advance amount on the basis of the engine speed detected by the crank angle sensor 13 and the charging efficiency of the engine.
Thus, the ECU 17 controls the valve timing so that the VVT controlled amount (variable) coincides with the required advance amount obtained from the two-dimensional map on the basis of the engine speed and the intake amount.
As mentioned above, the required advance amount is stored in the form of a deviation of the advance amount from the maximum retardation value employed as a reference, and signifies a required VVT control variable. Accordingly, if the required advance amount is zero, the ECU 17 carries out control so that the VVT control variable assumes zero, with the valve timing being set to the maximum retardation side.
Next, a description will be made hereinbelow of a valve timing detecting device.
The sensor plate 12 and the sensor plate 14 are axially fixed on the crank shaft and the cam shaft, respectively. Projections are formed on outer circumferences of the sensor plates 12, 14.
Furthermore, in the vicinity of the sensor plates 12, 14, the crank angle sensor 13 and the cam angle sensor 15 are located facing the outer circumferences thereof, respectively. The crank angle sensor 13 and the cam angle sensor 15 detect as variations of magnetic fields, the variations in the distance between the crank angle sensor 13 and the sensor plate 12 and the variations in the distance between the cam angle sensor 15 and the sensor plate 14, occurring with rotation of the sensor plates 12, 14 respectively.
Thus, with the rotation of the crank shaft and the cam shaft, the sensor plates 12, 14 rotate, and the crank angle sensor 13 and the cam angle sensor 15 sense the projections on the outer circumferences thereof to thereby detect a crank angle and a cam angle, respectively.
FIG. 16 is a flow chart showing the contents of the processing in a conventional valve timing control system for an internal combustion engine.
First, the operational flow starts at a step 1601 to check, on the basis of coolant temperature (cooling water temperature) in the internal combustion engine 1, whether the warming-up of the internal combustion engine 1 has been completed or not. If a decision in this step shows the completion of the warming-up, the operational flow advances to a step 1602 to set the valve overlap at an normal value.
In this instance, the term "valve overlap" means the overlap between the time that the intake valve is in the open condition and the time that the exhaust valve is in the open condition.
On the other hand, if the decision in the step 1601 indicates no completion of the warming-up, the operational flow proceeds to a step 1603 to set the valve overlap to a shorter value than ordinarily.
In this way, in the conventional valve timing control system for an internal combustion engine, the valve overlap is set to be short or small until the completion of the warming-up, while being set at the normal value after the completion of the warming-up.
In the conventional valve timing control system for an internal combustion engine, the reason why the valve overlap is set at a small value when the warming-up of the internal combustion engine is not completed, that is, when the internal combustion engine is cold, is that the combustion condition becomes unstable in this state.
If the valve overlap increases in the unstable combustion condition, due to the effects of the internal EGR, the residual exhaust gas increases within the cylinder and the combustion condition becomes further unstable because of the drop of the combustion temperature, which can lead to the occurrence of misfire. For this reason, commonly, control is implemented to set the valve overlap at a low value when an internal combustion engine is cold.
However, in the aforesaid internal combustion engine, the combustion condition can be unstable even after the completion of the warming-up.
For instance, at the start of the internal combustion engine, the fuel injection amount is increased in order to improve the starting performance. This control is done even at the starting after the completion of the warming-up, and immediately after the start, the combustion may be unstable by the influence of the extra fuel supply at the start.
In addition, in such a case, if the valve overlap is controlled to a value at an ordinary time, due to the increase in the internal EGR, the combustion temperature drops to cause the combustion condition to be more unstable, which results in the deterioration of the clarification of the exhaust gas and in the impairment of fuel consumption.